1. Field of the Invention
The present invention relates to a method and an equipment for producing a film, especially a method and an equipment for producing a film for an optical use.
2. Description Related to the Prior Art
In an optoelectronics field are used many polymer films which are produced in a solution casting method. The polymer film produced in the solution casting method is more excellent in an optical isotropy and a uniform thickness, and contains foreign materials less than a film produced in a melt-extrusion method. Therefore the polymer film is used, for example, as a protective film for a polarizing filter, a retardation film, a transparent conductive film, and the like. Especially among such polymer films, a cellulose acylate film is widely used since having transparency and adequate moisture permeability, high mechanical strength, and low dependency of a size stability on humidity and temperature. In the solution casting method, the polymers (such as cellulose acylate and the like) and the additives are dissolved to a solvent such that a dope may be obtained. Then the dope is cast from a die on a support, and peeled from the support when having a self-supporting property. The peeled film is dried in a drying process to obtain a film. The support is a metallic drum or a belt that is continuously moved in a cycle.
Further, recently, the above optoelectronics field shows noticeable progress, and it is required that the polymer film as one material thereof may have high function and multifunction. Accordingly, the polymer film must become thinner. In the solution casting method, a decompression chamber (corresponding to an aspiration chamber of the present invention) is provided near the dope between the die and the support so as to decompress in an area from a rear of the dope. Thus the decompression is made in an upstream side of the support from the cast dope between the die and the support, such that the thickness of the polymer film may become thinner. Further, also in the melt extrusion method, in order to make the film thinner, the film is produced from the molten polymer which is extruded from the die by aspirating the air in the same manner.
However, when the film becomes thinner, the production thereof becomes harder, and especially it becomes extremely harder to have a uniform thickness. For example, when the decompression is made in a rear side of the dope with the decompression chamber, the film thickness fluctuates in accordance with the pressure change in the decompression chamber. In this case, the distribution of the film thickness usually often shows cyclical pattern in lengthwise direction. The nonuniformity of the film thickness is a deadly defect of the product when the film thickness is at most 100 μm. Further, in order to decrease the fluctuation of the film thickness, the dope is diluted, or the drying speed of the film peeled from the support is made lower. Thus the film thickness is sometimes made uniform. However, the nonuniformity remains although reduced. Further, in the both cases, since the drying time becomes longer, the productivity becomes lower and the production cost becomes higher.
Accordingly, several proposal has been made about the method of producing the film from the dope with the decompression chamber, to prevent the generation of the nonuniformity of the film thickness. For example, in Japanese Patent Laid-open Publication No. 6-155494, an air inlet is formed at a position of at most 0.3 times of a width from each side edge of the decompression chamber. Through the air inlet, the air near the rear of the dope between the die and the support is aspirated into the decompression chamber under reduced pressure. In this method, the structure of the decompression chamber is the most adequate to reduce the wave-like nonuniformity of the film thickness.
Further, Japanese Patent Laid-Open Publication No. 10-272637 discloses a method of producing a film from a dope with at least two aspirating means. The dope is discharged from a slit of the die onto a surface of the moving support, and a casting film formed from the dope on the support is conveyed. In this method of producing the film, at least two aspirating means are used for preventing the movement of the applied point of the cast dope onto the substrate so as to increase the tightness of the casting film to the support.
However, in the method of the publication No. 6-155494, although the generation of defects of the film surface is reduced, there are no effects for reducing the thickness nonuniformity which is cyclically generated in the lengthwise direction. Further, in the publication No. 10-272637, the decompression chamber is disposed only in upstream from the applying point. In this structure, the movement of the applying point is not only reduced, but the fluctuation of pressure in the decompression chamber and the pipes thereto also enlarge the movement of the applying point.